The invention concerns a dual-fuel injector for an internal combustion engine, an internal combustion engine with such a dual-fuel injector, and a method for operating such an internal combustion engine.
Gaseous fuels are gaining importance in the large engine sector, in particular for stationary power-generating engines. In particular, because of its excellent availability and favorable emission potential especially in comparison with diesel fuel, natural gas is ideal for economic and environmentally-friendly engine operation. In gas engines which work on the diesel method (compression ignition), normally a small quantity of diesel is required as a starting aid for the subsequent gas diffusion combustion. The injection jet profiles of the two fuel systems (diesel and gas) should be oriented symmetrically in order to achieve a maximum overlap, in particular clearly assignable, of the two spray patterns. For this, dual-fuel injectors are known which have a first injection device for injection of a first fuel, wherein at least one second injection device is provided, arranged radially outwardly from the first injection device, for injection of a second fuel, wherein the second fuel and the first fuel are different. In particular, it is usual for a fuel gas to be used as the second fuel, wherein the first fuel is an ignition oil, preferably diesel. A first port is provided for supplying this first fuel, and a second port is provided for supplying the second fuel. A dual-fuel injector in which a plurality of gas metering valves as second injection devices are arranged coaxially to a centrally arranged diesel injector as the first injection device, is described for example in German patent application DE 10 2013 022 260 B3.
Such dual-fuel injectors have holes and volumes, in particular blind holes and injection holes, from which, at the end of an injection process after the injector needle has closed, fuel can expand at low pressure into the combustion chamber where it is no longer or only partially combusted. This is a problem in particular with gaseous fuels which expand very greatly on expansion, and in particular with natural gas where this effect leads to a high methane slip.